Thermal transfer printing is widely used in special applications such as in the printing of machine-readable bar codes on labels or directly on articles to be coded. The thermal transfer process employed by these printing methods provides great flexibility in generating images and allowing for broad variations in style, size and color of the printed images, typically from a single machine with a single thermal print head. Representative documentation in the area of thermal transfer printing includes the following patents:
U.S. Pat. No. 3,663,278, issued to J. H. Blose et al. on May 16, 1972, discloses a thermal transfer medium having a coating composition of cellulosic polymer, thermoplastic resin, plasticizer and a "sensible" material such as a dye or pigment.
U.S. Pat. No. 4,315,643, issued to Y. Tokunaga et al. on Feb. 16, 1982, discloses a thermal transfer element comprising a foundation, a color developing layer and a hot melt ink layer. The ink layer includes heat conductive material and a solid wax as a binder material.
U.S. Pat. No. 4,403,224, issued to R. C. Winowski on Sep. 6, 1983, discloses a surface recording layer comprising a resin binder, a pigment dispersed in the binder, and a smudge inhibitor incorporated into and dispersed throughout the surface recording layer, or applied to the surface recording layer as a separate coating.
U.S. Pat. No. 4,463,034, issued to Y. Tokunaga, et al. on Jul. 31, 1984, discloses a heat-sensitive magnetic transfer element having a hot melt or a solvent coating.
U.S. Pat. No. 4,523,207, issued to M. W. Lewis et al. on Jun. 11, 1985, discloses a multiple copy thermal record sheet which uses crystal violet lactone and a phenolic resin.
U.S. Pat. No. 4,628,000, issued to S. G. Talvalkar et al. on Dec. 9, 1986, discloses a thermal transfer formulation that includes an adhesive-plasticizer or sucrose benzoate transfer agent and a coloring material or pigment.
U.S. Pat. No. 4,687,701, issued to K. Knirsch et al. on Aug. 18, 1987, discloses a heat sensitive inked element using a blend of thermoplastic resins and waxes.
U.S. Pat. No. 4,698,268, issued to S. Ueyama on Oct. 6, 1987, discloses a heat resistant substrate and a heat-sensitive transferring ink layer. An overcoat layer may be formed on the ink layer.
U.S. Pat. No. 4,707,395, issued to S. Ueyama, et al., on Nov. 17, 1987, discloses a substrate, a heat-sensitive releasing layer, a coloring agent layer, and a heat-sensitive cohesive layer.
U.S. Pat. No. 4,777,079, issued to M. Nagamoto et al. on Oct. 11, 1988, discloses an image transfer type thermosensitive recording medium using thermosoftening resins and a coloring agent.
U.S. Pat. No. 4,778,729, issued to A. Mitsubishi on Oct. 18, 1988, discloses a heat transfer sheet comprising a hot melt ink layer on one surface of a film and a filling layer laminated on the ink layer.
U.S. Pat. No. 4,869,941, issued to Ohki on Sep. 26, 1989, discloses an imaged substrate with a protective layer laminated on the imaged surface.
U.S. Pat. No. 4,923,749, issued to Talvalkar on May 8, 1990, discloses a thermal transfer ribbon which comprises two layers, a thermal sensitive layer and a protective layer, both of which are water based.
U.S. Pat. No. 4,975,332, issued to Shini et al. on Dec. 4, 1990, discloses a recording medium for transfer printing comprising a base film, an adhesiveness improving layer, an electrically resistant layer and a heat sensitive transfer ink layer.
U.S. Pat. No. 4,983,446, issued to Taniguchi et al. on Jan. 8, 1991, describes a thermal image transfer recording medium which comprises as a main component, a saturated linear polyester resin.
U.S. Pat. No. 4,988,563, issued to Wehr on Jan. 29, 1991, discloses a thermal transfer ribbon having a thermal sensitive coating and a protective coating. The protective coating is a wax-copolymer mixture which reduces ribbon offset.
U.S. Pat. Nos. 5,128,308 and 5,248,652, issued to Talvalkar, each disclose a thermal transfer ribbon having a reactive dye which generates color when exposed to heat from a thermal transfer printer.
And, U.S. Pat. No. 5,240,781, issued to Obatta et al., discloses an ink ribbon for thermal transfer printers having a thermal transfer layer comprising a wax-like substance as a main component and a thermoplastic adhesive layer having a film forming property.
High speed thermal transfer printers such as "near edge," "true edge," "corner edge" and "Fethr.RTM." edge printers have been developed, wherein the thin film resistors are positioned right at the edge of the thermal print head, allowing rapid separation of the donor film from the receiving substrate after the thin film resistors are fired.
Conventional general purpose ribbons often cannot meet the requirements of high speed printers since the ribbon and receiving substrate are separated almost instantaneously after the thin film resistors are fired. There is little time for waxes and/or resins to melt/soften and flow onto the surface of the receiving substrate before the ribbon is separated from the receiving substrate. In conventional ribbons, the adhesion of the melted/softened material to the receiving substrate is typically lower than its adhesion to the supporting substrate of the ribbon at the time of separation with a high speed printer. As a result, the functioning thermal transfer layer is usually split and the transfer incomplete, resulting in light printed images where the functional layer is an ink layer.
To increase the speed of transfer of a functional layer to match the capability of high speed printers, waxes having a low melt temperature can be employed. A problem with this approach to utilizing the full capability of high speed printers is that the environmental stability of such ribbons decreases. As the melting point of the wax used to produce the ribbon decreases, the ribbon has a tendency to "block" wherein the coating transfers to the backside of the ribbon when wound onto itself. This blocking phenomenon tends to occur when the ribbon is subjected to temperatures in the range of 45.degree. to 55.degree. C. and when the ribbon is wound onto itself coating side in.
The thermal transfer layers of thermal transfer ribbons are typically of a thickness of 10 microns or less and preferably 5 microns or less, particularly for high speed printers. See for example, U.S. Pat. No. 4,547,088, column 2, lines 36-41; U.S. Pat. No. 4,938,617, column 4, lines 9-10; U.S. Pat. No. 4,744,685, column 3, lines 41-47; U.S. Pat. No. 4,870,727, column 17, lines 33-36; and U.S. Pat. No. 5,240,781, column 9, lines 51-53.
With these thin layers, the particle size of ribbon components such as pigments and other fillers are known to affect the resolution of the transferred image. See the Examples of U.S. Pat. No. 5,248,652 and U.S. Pat. No. 5,128,308 and see also U.S. Pat. No. 4,983,681 and U.S. Pat. No. 4,910,113. It is generally desirable to provide fine size pigments or other non melting components of the thermal transfer media.
In contrast to these typical ribbons Micke et al., U.S. Pat. No. 5,132,139 describes a thermal printing ribbon with multi strike capacity wherein the thermal transfer layer is preferably between 10 and 20 microns (see columns 7, line 21). For such thick thermal transfer layers, large size solid particles can be employed in the coating dispersion as is disclosed in column 5, lines 41-49.